The present invention generally relates to a surface acoustic wave device (hereinafter referred to as SAW device in abbreviation). More particularly, the invention concerns a SAW device capable of handling electric signals of high power and/or a large amplitude.
At present, the SAW device finds a wide range of utilization and application and is employed as a SAW filter adapted for transmission of high electric power or as a SAW resonator in which a surface wave of a large amplitude is continuously present as a standing wave. However, the SAW devices mentioned above suffer short-comings in that defects similar to hillocks, voids or the like due to electro-migration which take place in Al-metallized electrodes in semiconductor IC devices are produced in fine fingers of aluminum (Al) serving as SAW transmission/reception electrodes and reflector electrodes in the SAW devices, as is reported in "Thin Solid Films", 64, 9-15 (1979) (J. I. Latham et al), pp. 9-14 and "Transactions of The Institute of Electronics Communication Engineers of Japan", Vol. J67-C, pp. 278-285 (Mar., 1984). Under the circumstances, SAW resonators undergo unwanted phenomenon such as frequency deviation or shift from the resonance frequency. On the other hand, in the case of the SAW filter designed for high power transmission, there often arise failures such as short-circuit, wire breakage and the like due to the growth of hillocks, as is reported in "1983 IEEE Ultrasonic Symposium Proceedings", pp. 83-86 (1983). In particular, the second mentioned literature explains the generating mechanism of such failure as follows: "Strain in a substrate surface produced by the SAW induces an internal stress in the Al-thin film formed on the substrate surface, as a result of which Al-crystalline grain boundary migrates in a region where the stress exceeds a threshold, giving rise to creation of the voids and hillocks. The migration of the grain boundary is believed to be ascribable to such mechanism as experienced in the thermal cycle of ICs, as reported in IEEE Trans, Parts, Hybrids and Packaging, Vol. PHP-7, pp. 134-138, Sept. 1971". All of the literature mentioned above disclose that the failures due to Al-migration can be dealt with by adding a small amount (1 to 4 wt. %) of copper (Cu) to Al forming the conductors in the semiconductor ICs and that this method is effective for suppressing the migration.
However, as the frequency becomes higher, the strains due to SAW become greater even when the power for transmission and the amplitude remain unchanged. In the case of the SAW resonator designed for handling signals of frequencies higher than 300 MHz such as a SAW filter employed in a transmitter of a cellular radio telephone system of a 800 MHz-band and the like, an adequate useful life can not be assured because of occurrence of the above-mentioned migration in operation with high power and/or a large amplitude even with the Al-electrode having Cu added and formed through vacuum evaporation or EB evaporation disclosed in the literature enumerated above.
It is further noted that formation of Al-film containing an additive of Cu through the vacuum evaporation or EB evaporation tends to increase the hardness of film, making it difficult to employ the wire bonding which is another disadvantage. On the other hand, when a dry etching process is adopted for forming fine electrodes with a high precision for the purpose of enhancing the high frequency performance, the electrodes are susceptible to corrosion or the like (e.g., due to the inclusion of Cl), givin rise to a problem that the yield is degraded significantly.
Another method of suppressing the electromigration is described in J. Appl. Phys., 49 (7), pp. 4083-4093, July 1987. The method described in this literature is featured as follows. More particularly, the wiring in a semiconductor IC is of a multilayer comprising a first layer of an Al film which includes Cu as an impurity and a second layer of, for example, a hafnium (Hf) film which is exclusive of Al and much thinner than the first Al film layer.
With the above prior art, when a Hf film is exemplarily used as the second layer, a good migration-proof characteristic can be obtained but, on the other hand, Hf molecules in the second layer easily diffuse into the Al of the first layer. Therefore, in an application wherein the semiconductor IC with the multilayer wiring is operated for a long period of time at a temperature of about 80.degree. C., the first layer Al film and the Hf molecules in the second layer are accelerated for alloying together, resulting in a considerable increase in resistivity of the overall film. Accordingly, a SAW device incorporating such a wiring structure suffers from a drastic increase in power loss and can not sustain its utilization as required.
In co-pending application (A. Yuhara et al) Serial No. 2,286 (based on Japanese Patent Application Nos. 3428/86 and 46138/86) filed Jan. 12, 1987 and assigned to the assignee of the present invention, a SAW device is disclosed in which electrodes are formed on a piezoelectric substrate by sputtering and/or the electrodes contain an additive of Cu, Ti, Zn, Mg, Fe, Ni, Cr, Ga, Ge, Sn, Pd or Ta.